(1) Field of the Invention
The present invention relates to an operating method polymerizing .alpha.-olefin by vapor phase polymerization, and more particularly to an operating method by which production of a sheet-like polymer in an area close to the inner wall of its reactor in the case when .alpha.-olefin is polymerized or copolymerized by the use of a vapor phase fluidized bed may be reduced, and unstable reaction of such polymerization may be prevented from occurring.
(2) Description of the Related Art
It has been disclosed, for example, in EP-A No. 224479 that an absolute value of electrostatic potential during a polymerization reaction has to be maintained in zero condition, in other words, a polarity of existing voltage has to be kept neutral in order to prevent production of sheet-like polymers in an area close to the inner wall of its reactor.
In the event of polymerizing .alpha.-olefin by using a vapor phase fluidized bed, there is such a case where sheet-like polymers are produced in an area close to the inner wall of its reactor especially at the beginning of the polymerization, and then a port for drawing out polymer and the downstream piping therefor are clogged with such sheet-like polymers so that the polymerization reaction cannot substantially be continued anymore. Such production of sheet-like polymers is frequently observed within a period after a solid catalyst component and an organic aluminum compound are supplied into a reactor in a state where substantially no polymerization occurs, then, the polymerization is initiated, and before about twenty times higher volume of a polymer than that of a reacting zone of its fluidized bed are produced. Thereafter, production of sheet-like polymers decreases comparatively after a condition of the reaction transfers to its steady state.
If the polyolefin particles in a reactor are electrified, it is known that such electrified particles adhere to another substance by electrostatic forces. Accordingly, it is supposed that there is a relationship between the electrified polyolefin particles in the reactor and production of sheet-like polymers in an area close to the inner wall of the reactor.
In this connection, first, behavior of electrified polyolefin particles in a vapor phase fluidized bed as well as production of sheet-like polymers will be described in detail hereinbelow.
In a vapor phase fluidized bed reactor, electric charge arises due to contact and/or friction between polymer particles as well as between polymer particles and the inner wall of the reactor so that these polymer particles are electrified, respectively. When these particles possess a high amount of electric charge and the number of particles electrified increases, such electrified polymer particles come to adhere to the inner wall of the reactor due to Coulomb forces. The velocity of the gas stream becomes slow in an area close to the inside wall of the vapor phase fluidized bed reactor so electrified polymer particles adhere much more easily to the inside wall of the reactor.
However, such adhering of electrified polymer particles to the inner wall of a reactor does not directly relate to production of sheet-like polymers. Even if polymer particles have adhered to the inside wall of a reactor, when a temperature in a layer of such adhered polymer particles does not exceed a melting point of the resulting polymer, no sheet-like polymer is produced. This result as described above is obtained in such a case where a concentration of catalyst used is low in a relevant area, in other words, an amount of reaction is small in the area and as a consequence, associated heat of polymerization can be removed.
On the other hand, when electrified polymer particles adhere to the inside wall of a reactor due to Coulomb forces and a concentration of the catalyst used in a layer of the adhered polymer particles is high, a polymerization reaction proceeds, and as a result it becomes difficult to remove the heat of the polymerization. Consequently, the temperature in the layer of adhered polymer particles rises. Finally, such temperature reaches one which exceeds the melting point of the polymer, and the polyolefin particles fuse together to form a sheet-like polymer. While the expression "concentration of catalyst" has been used herein, in reality, when a catalyst is supplied into a reactor, polymerization begins promptly to form polymer particles in the presence of the catalyst, and these particles continue further polymerization. More specifically, even if a concentration of catalyst supplied is low in a relevant area, production of sheet-like polymers is observed after all in the case where a concentration of polyolefin particles having polymerizing activity is high in the area. For this reason, a more pertinent expression might be "concentration of active sites for polymerization" or "amount of reaction per unit volume", but not "concentration of catalyst". In any event, the concentration in this case means the concentration of solid catalyst component which have been supplied to a reactor as well as the concentration of the resulting polyolefin particles having polymerizing activity. In this respect, the term "concentration of catalyst" does not mean simply hereunder that of the catalyst, but also includes a concentration of polyolefin particles having polymerizing activity.
It is, however, to be noted that while such polymerization as described above proceeds under these circumstances, production of sheet-like polymers requires further accumulation of heat of polymerization, in other words, it is required for the production of sheet-like polymers that such polyolefin particles adhere to the inside wall of a reactor in the form of a layer with a certain degree or more of thickness. Accordingly, there is no production of sheetlike polymers with such thickness of a thin layer of the adhered polyolefin particles after all.
With respect to such adherence of polyolefin particles in the form of a layer with a certain degree of thickness, to be noted is coexistence of particles which have been electrified positively and other particles which have been electrified negatively in the electrified polymer particle group in a reactor.
For the following explanation, polyolefin particles which have been merely electrified either positively or negatively as a group will be assumed herein. As a matter of course, such particles may adhere onto the inner wall of a reactor to form the first layer thereof due to Coulomb forces. However, even if other electrified polymer particles approach the first layer, further adherence never occurs as a result of action of repulsion force due to Coulomb forces because these particles have the same polarity of electric charge. Thus, the polymer particles adhere to the reactor inside wall with merely a thickness of a single layer in the case where all the existing particles are ones which have been electrified in only the same polarity. From the above description, it will be appreciated that coexistence of particles which have been electrified positively and ones which have been electrified negatively as a group of polyolefin particles is a requirement for adherence of such polymer particles in a reactor on the inside wall of a reactor with a layer having a certain degree of thickness. In addition, it is to be noted that since polymer is an insulating material, it is hard to occur the transfer electric charge in the polymer and its surface. However, when such polyolefin particles are in contact with a conductive material such as an electrode which will be described later, at least a part of the electric charge at the site being in the contact state transfers immediately to the electrode. Accordingly, there may be such a case where one polymer particle includes a plural number of electric charges. In this case, the plural number of electric charges may be different from one another in their signs (polarities). More specifically, there is a case where a positive and negative electric charge coexist within one polymer particle.
From the fact that production of sheet-like polymers is observed in an actual vapor phase polymerization of polyolefin, it is considered that an electrified state of the polyolefin particles in vapor phase polymerization is also a complicated electrified state in that both a positive and a negative electric coexist in one polymer particle as described above.
Therefore, it is important to have a good grasp of an electrified state of polymer particles such as coexistence of positive and negative electric charges in view of a countermeasure on how to cope with adherence of polymer particles to the inner wall of a reactor which might lead to production of sheet-like polymers.
Heretofore, there have been two methods for measuring an electrified state of particles and both of them are each for measuring electrostatic potential One method is a (non-contact way) method for measuring electrostatic potential of particles without being in contact with the particles which are the object to be measured. The other is a (contact way) method for measuring electrostatic potential with contacting of the particles to be measured. The term "electrostatic potential" used herein means one which is due to particles electrified and in this case, potential of the earth is considered to be zero as the reference potential. This may be also called by the term "electrostatic voltage", and which is hereinafter referred to simply as "potential".
In the event where a disaster due to discharge of static electricity of particles are prevented from occurring, potential of the particles is a very important factor. This is because discharge of electricity is established to lead a cause for disaster, when potential of electrified particles exceeds electric breakdown voltage between particles and the earth. In other words, electric field strength of the electrified particles exceeds electric breakdown strength between particles and the earth. In such a case, a conventional method for measuring potential is optimum as a method for measuring a state of electrified particles.
However, the conventional method for measuring potential is not suitable for one for measuring a state of polymer particles electrified in a complicated situation. This will be amplified hereinbelow by exemplifying the conventional measuring method.
As a non-contact measuring method, there is, for example, a manner wherein electric field due to electric charge of an object to be measured which has been electrified is measured by using an electrostatic field meter, and potential of the object to be measured is determined by the electrostatic field measured and a distance extending therefrom to the object to be measured. The above described method will be described further by referring to FIG. 1, wherein +q designates an amount of electric charge in a charged particle 1. In this connection, FIG. 1 illustrates an example of a non-contact way potential measuring method. In general, since an electrostatic field meter 2 is connected to the earth ground 3, when strength of electric field and a distance extending therefrom to an object to be measured are expressed by E [V/m] and d [m], respectively, an electric potential difference, i.e. potential V [V] is determined as follows: EQU V=E.times.d
However, in the case where an electrified state of polymer particles (group) is measured in accordance with a noncontact way potential measuring method, there is such a problem that measurements may be significantly out of order if the polymer particles being the object to be measured come to be in contact with a measuring section of the electrostatic field meter even though the meter is non-contact type. A cause for the problem is electric charge transferred or produced as a result of the contact between the polymer particles and the measuring section of the electrostatic field meter used. For this reason, such noncontact type meter is not generally used for measuring potential of particles.
On the other hand, a conventional contact way measuring method is one wherein an electrometer is connected with an electrode which is in contact with the polymer particles being the object to be measured to determine potential as described in, for example, the above-mentioned EP-A No. 224479. Such contact way measuring method is one for measuring potential of the electrode to which has been transferred electric charge from the polymer particles. FIG. 2 illustrates a contact way potential measuring method. As shown in FIG. 2, an electrometer 6 is connected to an electrode 5 which is inserted into a reactor 4. Further, the electrometer 6 is also connected to earth ground 3. The charged particles fluidized in a reactor (not shown) are brought into contact with an electrode.
In this method, the electric charge which has been transferred from polymer particles to an electrode accumulates in the electrostatic capacitor which is formed between the earth ground and a measuring system composed of electrodes, an electrometer, wirings for the electrodes and electrometer, and the like components so that potential in the measuring system changes. When the potential of an electrode changes, an amount of electric charge transferred from polymer particles changes in even the case where such polymer particles having the same amount of electric charge comes in contact with the electrode. In addition, frequency of contact of polymer particles with the electrode varies under the influence of Coulomb force. Furthermore, an amount of electric charge leaking from the measuring system varies also in accordance with changes in potential of the measuring system. Finally, potential of the electrode in the case when equilibrium is established by both the amount of electric charge transferred from the polymer particles to the electrode and the amount of electric charge leaking from the measuring system is obtained as a measured value.
It is, however, necessary for paying attention to the fact that the measured value of potential thus obtained is such potential of an electrode in the event where equilibrium is established by both the amount of electric charge transferred from the polymer particles to the electrode and the amount of electric charge leaking from the measuring system, but it is never the potential of polymer particles (group) existing nearby the electrode after all.
Such measured potential value of polymer particles (group) is affected by electrostatic capacity between the polymer particles (group) and ground. More specifically, even if polymer particles (group) have the same amount of electrification, when the electrostatic capacity appearing with respect to ground changes (electrostatic capacity varies also due to a shape, volume and a correlative relationship in position of a reactor), measured potential value of the polymer particles (group) changes accordingly.
Now, what is meant by measurements in the present method will be described in conjunction with the circuit diagram shown in FIG. 3 wherein C [F] designates electrostatic capacity of a capacitor formed between the measuring system and earth ground, and R [.OMEGA.] designates insulation resistance of the measuring system. The circuit diagram shown in FIG. 3 is electrically equivalent to the diagram of FIG. 2. In this case, each polymer particle has a constant amount of positive electric charge, and it is assumed that the initial potential of an electrode is zero.
With accumulation of electric charge as a result of transferring of the same from polymer particles to the capacitor formed between the measuring system and earth ground, potential of the electrode increases. When an amount of electric charge accumulated in the capacitor is designated by q [C], potential V [V] of the electrode is expressed by V=q/C.
With increase of the potential of the electrode, the repulsive force acting between the electrode and the polymer particles increases so that the polymer particles become difficult to be in contact with the electrode. In addition, such amount of electric charge which transfers to the electrode in the case when the polymer particles are in contact therewith decreases also. For this reason, an amount of electric charge transferring to the electrode per unit time, i.e. current I.sub.in derived from the electrode varies dependent on the potential V of the electrode. This may be expressed by I.sub.in =f(V) in accordance with a function f(V).
Leakage current I.sub.out from the measuring system may also be a function of the potential of the electrode, and this is expressed by I.sub.out =V/R in accordance with Ohm's law. Finally, a measured value of potential becomes the potential V of the electrode in the case where I.sub.in -I.sub.out =0, i.e. EQU f(V)=V/R (I)
is valid.
In this case, the function f(V) expressing transfer of electric charge from polymer particles to an electrode having potential V is fixed dependent upon the electrode used for the measurement, types of polymer, an electrified state of the polymer particles.
In the case where Equation I is in such a form which includes no variable other than the function f(V) and the potential V, when the electrode and a type of the polymer are fixed, the measured value V is determined uniquely with respect to an electrified state of polymer particles. However, Equation I includes an insulation resistance R of the measuring system other than the function f(V). This means that the measured value, i.e. the potential V of electrode which satisfies Equation I is affected by an insulation resistance of the measuring system. Namely, this means that when the insulation resistance of the measuring system is not kept constant, the measured value V is not uniquely determined with respect to the electrified state of polymer particles even if the electrode and the types of polymer are fixed.
In other words, it may be concluded that measured values cannot be directly compared with each other in such a contact way method for measuring potential as mentioned above so far as its measuring system is specified, and its condition is kept constant. Accordingly, it may be said that direct comparison of the resulting measured values is extremely difficult in the contact way method for measuring potential described in the aforesaid EP-A No. 224479 from the theoretical point of view.
In either measuring method of contact or non-contact way as described above, measurements are obtained in the form of potential or voltage. Such measurements of potential are, however, based on a difference between positive and negative electric charge as its polymer particle group. Accordingly, it may be concluded that these methods are not suitable for one for measuring a complicated electrified state of a polymer particle group in which there is coexistence of positive and negative electric charge. Besides balance between positive and negative electric charge is important with respect to adherence of the polymer particles to the inside wall of a reactor.
In this connection, when, for example, such polymer particle groups composed of coexisting electrically equivalent numbers of polymer particle groups each of which is positively and negatively electrified are imagined and these polymer particle groups are measured by a method for measuring potential, the resulting measured value is theoretically zero as potential. This is because a difference between positive and negative electric charge is zero irrespective of the aforesaid contact and non-contact way methods.
According to the aforesaid EP-A No. 224479, prevention of producing sheet-like polymers is attained by keeping its electrostatic potential neutral. Accordingly, it may be considered that there is no sheet-like polymer produced in the case as described above where electrically equivalent number of polymer particle groups coexist so that its electrostatic potential is neutral. However, since such polymer particles each of which is positively and negatively electrified coexist, it is highly possible that thick adherence of polymer particles onto the inside wall of a reactor as mentioned above will occur in this case.
As described above, it is uncertain and difficult to accomplish prevention of producing a sheet-like polymer in the case of vapor phase polymerization of .alpha.-olefin in accordance with measurement of potential.
Meanwhile a mechanism for generating static electricity has not yet been elucidated in general, and there is the same tendency as to vapor phase polymerization of polyolefin.
Accordingly, fundamental elucidation of a mechanism for generating static electricity and countermeasures for preventing generation of static electricity based on such elucidation are inevitably insufficient and incomplete. Consequently, we must study a manner for dealing with and solving the problem of generating static electricity on the premise that polymer particles are electrified.
On the other hand, there is such a case where minute current flowing through an electrode is measured for determining an amount of electric charge in a field relating to electrostatic phenomena. This is because it may also be considered that current is differential of an amount of electric charge transferred with respect to time. Therefore, minute current is measured, and the measured value is integrated with time, whereby a minute amount of electric charge transferred can be calculated. In case of measuring an amount of electric charge, generally a direction along which current flows, i.e. a direction along which electric charge transfers and measurements of current do not change in a short period of time.
On the contrary, in the case where a direction along which electric charge transfers and a value of current itself vary remarkably in a short period of time, it is difficult to obtain an effective value as an amount of electric charge even if a determined current value is integrated with time. Thus, heretofore utilization of a method for measuring current has been limited to such a case where a direction along which electric charge transfers and measurements of current vary slightly within a short period of time.
We noted this measuring method by which transfer of minute electric charge can be determined, and studied applying the measuring method to measurement of an electrified state of polymer particles.
In case of electricity served for daily use, mutual conversion of current and voltage is easy as is apparent from the fact that a typical voltmeter is composed of an ammeter and a resistor having a high resistance value. This is because a relationship between voltage and current has been formulated through the mediation of resistance (impedance).
In general, however, mutual conversion of current measurement values according to the method used in the present invention to those of conventional potential and electrostatic voltage is impossible. This is because these respective measurements are simply in the form of current or voltage as a manner for expressing an electrified state of polymer particles, and their measuring principles are quite different from one another.